Bokeh

Introduction:

Bokeh comes from the Japanese word Boke meaning
blur. It is used as a photographic term to describe the rendering of
out-of-focus areas of images. Bokeh refers to the aesthetic qualities of
this rendering and varies from lens to lens.

Bokeh:

Bokeh tends to come into play when you shoot with a
fairly wide open aperture. This creates a lot of out-of-focus points of
light in your image. Bokeh refers to how these points are rendered.

Bokeh has been referred to as good and bad. These terms are quite
general, but it is assumed that lenses that produce smooth blurred
out-of-focus areas have good Bokeh, while lenses that create harsh and
noticeable out-of-focus points of light are referred to as producing a
bad Bokeh. When you see bad Bokeh, you are essentially seeing large
circles of confusion. When you see good Bokeh, these circles flow and
blend into one another and are not visible.

You need to worry about Bokeh if you shoot a lot of portraits, macro,
use a long range telephoto lens, or shoot with apertures wide open. In
photography it is generally preferred to have in-focus foregrounds and
out-of-focus backgrounds.

This is an example of
good Bokeh.
Notice how the background is very smooth and blends together. In an
image with bad Bokeh you would be able to see obvious circles and the
background would look quite harsh.

This is an example of when
Bokeh starts to get worse and more noticeable. The out-of-focus
background now has some apparent circles of confusion also called blur
circles. These would get more pronounced and harsh with a lens with bad
Bokeh.

Bokeh Types:

A point of light in focus will yield a point on the
imaging plane. A point of light out-of-focus projects a cone of light
onto your sensor (instead of a point). Since all lenses contain some
sort of spherical aberration, these out-of-focus points are rendered
differently from lens to lens.

This
is a very magnified example of bad Bokeh. This is how out of focus
points of light will be rendered. The edge is very defined and when a
background is filled with this type of Bokeh it is very harsh. Mirror
lenses tend to produce this type of Bokeh. Here spherical aberration is
working against the image.

This
is neutral Bokeh. This is the result of an optically perfect lens with
no spherical aberrations, which renders the light in a blur circle
evenly all around. There is no such lens, but some come close. This is
not very good for a pleasing and smooth background either, but it's
better than the type above.

This
is an example of good Bokeh. The out-of-focus point of light is rendered
very gradually. When a background is filled with this type of Bokeh it is
very soft and flows together quite well. Here spherical aberration is
for the image.

Be aware that most modern lenses are designed to be
as near perfection as possible and will often yield results that
resemble neutral Bokeh. Good Bokeh is proof of lens imperfections though
it will create better backgrounds. You can't have an optically perfect
lens that gives good Bokeh...no such thing. Some lenses, especially
macro and long telephoto are designed to produce images with good Bokeh
since the occurrence of bad Bokeh can easily ruin these types of images.
Keep in mind that just because a lens produces good Bokeh, does not mean
it's a bad lens.

Think about Bokeh when you are shopping for a lens
that will be used with the aperture wide open, especially in macro and
long telephoto shots.